This application relates to chitooligosaccharides that lack a lipid moiety, and to methods of using such chitooligosaccharides to promote growth and/or development of non-leguminous plants.
Plants associate with a wide range of microorganisms that facilitate the acquisition of nutrients and protect them against biotic and abiotic stresses. For example, interactions with arbuscular mycorrhizal (AM) fungi are widespread in land plants, and this association aids in the uptake of nutrients from the soil (Harrison, M. J. (2005), Annu. Rev. Microbiol. 59: 19-42). Because AM fungi are obligate symbionts, little is known about the molecular and genetic basis of this symbiosis.
Much of what is known about AM fungi/plant symbiosis has come from studies of the symbiotic association between plants and nitrogen-fixing rhizobium bacteria, which most notably results in the formation of nitrogen-fixing nodules. Unlike the association between AM fungi and plants, the rhizobium bacteria symbiosis is restricted to specific groups of plants, primarily legumes (Soltis, D. E., et al. (1995), Proc. Natl. Acad. Sci. USA 92: 2647-2651). However, both interactions are similar in that they require chemical communication facilitated by the production of diffusible signals by the symbiont (Oldroyd, G. E. (2013), Nat. Rev. Microbiol. 11: 252-263).
Rhizobial bacteria signal to legumes with Nod factors, which are lipochitooligosaccharides (LCOs) containing a chitin backbone substituted with a lipid moiety, an N-acyl group, and a number of additional groups that vary between Nod factors produced by different species of rhizobia (Dénarié, J. et al. (1996), Annu. Rev. Biochem. 65: 503-535). Nod factor perception utilizes a signalling pathway that is also involved in the establishment of mycorrhizal associations (Oldroyd, G. E. (2013)).
AM fungi also produce diffusible signals that are recognized by the host plant via the common symbiosis signalling pathway. Research suggests that at least two different mycorrhizal signals are active on Medicago truncatula (Chabaud, M., et al. (2011), New Phytol. 189: 347-355). Similarly, work in rice (Oryza sativa) demonstrates mycorrhizal signalling that is both dependent and independent of the common symbiosis signalling pathway (Gutjahr, C., et al. (2008), Plant Cell 20: 2989-3005).
The AM fungus Rhizophagus irregularis produces LCOs (Maillet, F., et al. (2011), Nature 469: 58-63), some of which are sulfated, resulting in a structure very similar to the Nod factor produced by Sinorhizobium meliloti, the symbiont of M. truncatula. These Myc-LCOs activate responses in M. truncatula similar to those activated by Nod factor, including the promotion of lateral root outgrowth. Consistent with these findings, U.S. Patent Publication No. 2011/0301032 discloses a method of stimulating a plant by contacting the plant with Myc-LCOs and variants thereof, wherein the variants all retain a lipid moiety.
The lipid moieties incorporated into the LCOs disclosed by, e.g., U.S. Patent Publication No. 2011/0301032, make it difficult and/or expensive to synthesize large quantities of such compounds from commonly available chitooligosaccharide source materials. Furthermore, the lipid moieties decrease the solubility of such compounds in water, increasing the challenges associated with dissolving the compounds in aqueous solutions for scaled up application to seeds, seedlings, or plants. Accordingly, there is a need in the art for alternate compositions and methods for stimulating plant growth and/or development that do not have these disadvantages in large-scale applications.